1. Field of the Invention
The present invention relates to the field of analog-to-digital conversion and, more particularly, to a technique for detecting and suppressing overload conditions in a delta-sigma analog-to-digital converter.
2. Background of the Related Art
The general technique of providing analog-to-digital (A/D) or digital-to-analog (D/A) conversion of signals is well known in the art. Generally, the sampling rate required to sample an analog signal for A/D conversion must be twice the highest frequency component being sampled. This rate is known as the Nyquist rate. More recently, oversampling methods have been utilized for A/D and D/A conversion. In an oversampling type of converter, the sampling rate is much higher than the Nyquist rate. An oversampling technique is described in a reference titled "Oversampling Methods for A/D and D/A Conversion;" by James C. Candy and Gabor C. Temes; IEEE; pp. 1-25; 1992.
An advantage of using the oversampling technique is in the precision provided by the converter. With converters operating under the Nyquist rate for sampling, a certain amount of precision is required for the conversion. For example, in converting an analog signal into a 16-bit digital format, 16-bit precision is required. Accordingly, circuits will need to be designed having components which will meet this precision. In many instances, closely trimmed circuit components or precision matching (or compensating) circuits are required to meet the precision.
However, when sampling at a rate much higher than the required Nyquist sampling rate, the oversampling technique permits circuit precision to be relaxed significantly. For example, if the above A/D 16-bit oversampling converter implements an oversampling modulator, the modulator output can be a single bit output. The circuit precision needs only to meet this 1-bit output. Accordingly, closely trimmed circuit components are generally not needed. Additionally, 1-bit precision can be readily met by current generation CMOS (complementary-metal-oxide-semiconductor) components.
The disadvantage in using the oversampling technique is the added requirement that the output now needs to be reduced to the standard Nyquist rate at the eventual output of the converter. That is, the higher sampling rate now needs to be returned to the Nyquist rate. In the above 16-bit example, a multiple number of the 1-bit outputs will need to be combined to form a single 16-bit output, which output is equivalent to the 16-bit output from the Nyquist rate converter. However, the oversampling technique is preferred in many applications, since the cost savings in using less precise circuit components outweigh the additional digital signal processing needed at the back end of the converter.
One well known type of oversampling A/D conversion uses a modulator commonly referred to as a delta-sigma modulator. In a A/D converter (ADC) using a delta-sigma modulator, an integrator and a comparator are utilized at the front end of the converter to provide the quantization of the analog signal. Then, a low-pass filter and a decimator are utilized for digital signal processing to provide a corresponding digital signal at the Nyquist rate. However, the circuit precision of the analog circuitry can be relaxed, due to the use of the higher sampling rate. It is also the practice to design the delta-sigma modulators for higher than the first order of operation. Higher order delta-sigma modulators are desirable, since lower sampling rates can be utilized to obtain the same precision as operating the modulator at a lower order but with higher sampling rates. See for example, "A Stereo 16-Bit Delta-Sigma A/D Converter for Digital Audio;" by D. R. Welland et al.; Journal of the Audio Engineering Society, vol. 37, pp. 476-486; June 1989; and "A Higher Order Topology for Interpolative Modulators for Oversampling A/D Converters;" by Kirk C. H. Chao et al.; IEEE Trans. Circuits and Sys., vol. CAS-37, pp. 309-318; March 1990.
For proper modulator operation, it is desirable for the modulator to respond linearly to the input signal amplitude. However, it is also known that modulators can experience distortion near the peak amplitude levels of the input signal. This is especially true, if the full-scale digital output signal is equivalent to the full-scale analog input signal. Ideally, the clipping action of the digital decimation filter should occur at the peak modulator input signal amplitude, so that any distortion at the output is limited to a range of voltages beyond the specified maximum amplitude of the input signal. In reality, the overload point, where the modulator performance degrades suddenly, can vary from the desired operating point due to various circuit tolerances and ambient factors. That is, the sudden overload action may commence to occur prior to the peak input signal level, which limits the performance of the modulator.
In order to address the occurrence of such an overload condition in a delta-sigma modulator, one scheme utilizes a gain scaling technique to adjust the gain of the modulator stage. See for example, U.S. Pat. No. 4,851,841. In this technique, the gain of the modulator is scaled to provide an effective feedback reference voltage that has a value greater than the specified maximum input voltage and the gain is readjusted at the decimation filter.
It is to be noted that the overload condition is different than an unstable condition. In an overload condition, the modulator experiences a degraded signal-to-noise ratio when the input amplitude exceeds a certain value, but the modulator can recover when the overload condition is removed, typically by the reduction of the signal amplitude. Instability is also a function of the amplitude of the input signal, but in this instance (unlike the overload condition), the modulator generally cannot recover from an unstable behavior with the reduction of the input signal. The recovery is usually achieved by resetting the modulator.
The present invention is a different scheme to address the overload condition, in which the distortions encountered near peak input signal amplitude levels are compensated.